Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

摘要

A combination of optical feedback self-locking of a continuous-wavedistributed feedback diode laser to a V-shaped high finesse cavity, laser phasemodulation at a frequency equal to the free spectral range of the V-cavity anddetection of the transmitted laser beam at this high modulation frequency isdescribed for possible application in cavity-enhanced absorptionspectroscopy.In order to estimate an absorbance baseline noise of laserintensity and frequency modulated light triplet passed through the V-cavity inopen air, a 1.5-cm long optical cell filled by C2H2 at low pressure was placedbehind the cavity output mirror. The performance of the setup was evaluatedfrom the experimental bandwidth normalised relative intensity noise on thecavity output and the frequency modulation absorption signals induced by C2H2absorption in the 1.5-cm cell. From these data we estimate that thenoise-equivalent absorption sensitivity of 2.1*10^-11 cm^-1 Hz^-1/2 by a factorof 11.7 above a shot-noise limit can be achieved for C2H2 absorption spectraextracted from the heterodyne beat signals recorded at the transmission maximaintensity peaks of the successive TEM00 resonances.